1. Field of the Invention
The present invention relates to a fixing apparatus mounted on a copying machine or a printer, and in particular, it relates to a heater, a sleeve having its inner peripheral surface contacted with a heater, and a backup member for forming a fixing nip portion with the heater through the sleeve.
2. Description of the Related Art
In general, as a fixing apparatus mounted on a copying machine or a printer, a fixing apparatus of a film system has been put to practical use, which comprises a heater made of ceramic, a fixing film of material such as polyimide, stainless, and the like having its inner peripheral surface contacted with this heater, and a pressure roller for forming a fixing nip portion with the heater through the fixing film.
As one mode of this fixing apparatus of the film system, there is an apparatus providing an elastic layer such as a silicon rubber and the like for the fixing film. Due to the provision of the elastic layer for the fixing film, a toner image on the recording material can be fixed as though enwrapped. Consequently, this fixing apparatus is mainly used as a fixing apparatus mounted on a full-color printer.
Incidentally, the fixing film having an elastic layer is worse in heat transfer property of the fixing film than the fixing film having no elastic layer, and has such characteristic that heat of the heater is hard to travel from the inner surface to the front surface of the fixing film. Hence, there is a problem that, similarly to the case where the fixing film having no elastic layer is used, it is difficult to manage the temperature of the fixing nip portion to become the temperature suitable for fixing the toner by a temperature management method such as controlling current supply to the heater such that the temperature of the heater is detected and this detected temperature maintains at a set temperature of the fixing time period.
Hence, the fixing apparatus using the fixing film having the elastic layer adopts a temperature management method of controlling current supply to the heater such that the temperature of the fixing film is detected by a first temperature detection element and this detected temperature maintains at a set temperature of the fixing time period, so that the temperature of the fixing nip portion becomes a temperature suitable for the fixing of the toner. Further, by installing a second temperature detection element for detecting the temperature of the heater, an abnormal temperature rising of the heater is coped with. For example, when the detected temperature of the second temperature detection element exceeds a heat-resistance temperature of a heater holder, a control to shut off the current supply to the heater is performed.
By the way, in the recent copying machine or printer, a variety of media (recording material) used for the printer is diversified. Hence, to cope with a variety of media, the fixing apparatus must also set fixing conditions corresponding to the media.
As one of means for changing the fixing conditions, there is a method of changing a pressure applied to the fixing nip portion. For example, countermeasures can be considered where when print is made on an envelope, the pressure applied to the fixing nip portion is lowered than when print is made on a plain paper, thereby suppressing the generation of creases on the envelope. As a constitutional example making it possible to change an applied pressure of the fixing apparatus in this way, there exist Japanese Patent Application Laid-Open No. H6-11993 and Japanese Patent Application Laid-Open No. H10-282828.
Further, a technology for changing the fixing temperature when the applied pressure of the fixing apparatus is changed is also proposed. As such an example, for example, there exist Japanese Patent Application Laid-Open No. H02-132481 and Japanese Patent Application Laid-Open No. 2004-279702.
However, when the pressure applied to the fixing nip portion is lowered, the width in a recording material conveying direction of the fixing nip portion is narrowed. Hence, a contact area with the fixing film and the pressure roller becomes narrow, and efficiency of heat transfer from the fixing film to the pressure roller is lowered. Further, the efficiency of heat transfer from the heater to the fixing film is also lowered.
Hence, when the current supply is started for the heater in a state in which a pressure applied to the fixing nip portion is lowered (reduced pressure state), comparing with the case where the current supply is made without lowering the pressure (in case the input power is presumed to be the same), a temperature rising speed of the heater becomes fast. FIG. 5 shows a change of detected temperatures of a main thermistor (a first temperature detection element) and a sub-thermistor (a second temperature detection element) when the fixing nip portion is in a normal pressure state and in a reduced pressure state.
Particularly, when the pressure applied to the fixing nip portion is lowered and the current supply to the heater is started in a state in which the fixing apparatus is warmed up, there are often the cases where the heater reaches a high-temperature faster than the reaction of the second temperature detection element due to steep temperature rising of the heater. In such a case, melting of the heater holder made of resin is likely to occur. When the heater holder is melt, the pressure applied to the fixing nip portion is off its balance, thereby inviting negative effects such as non-uniformity of luster of an image and the like.